When using supported catalyst systems in a gas phase process, it is often necessary to provide them as free flowing powders. In case the catalyst is prepared by a suspension method, drying of the catalyst is necessary after supportation of the catalytic active component and cocatalyst on a support material. Drying of a wet mud, which means removal of solvent to obtain a free flowing powder after a possible filtration step is commonly carried out in the art by applying vacuum or by feeding an inert gas that carries away the solvent molecules; both methods are described in literature.
According to the first method, the drying of a catalyst system is generally performed in vacuum, by applying a pressure of 2-20 kPa. Nevertheless, depending on the amount of solvent and the nature of the polymer, this operation may take unacceptably long times at industrial level; the application of vacuum to remove the solvent may require a few hours at a laboratory level and full days in commercial plants.
Therefore, at industrial level, for reducing the drying time the drying step under vacuum has to be carried out at elevated temperatures, of at least about 50° C. With defined types of catalysts, such as late transition metal catalysts, this method, however, leads to catalyst systems having decreased activity.
The second methodology used in the art to dry a supported catalyst system comprises the use of an inert gas carrying away the solvent molecules. While this method is more careful and gives catalyst system particles with better flowability, it is much less effective than applying vacuum.
Therefore, it is felt the need in the art for a method for drying a catalyst mud while maintaining a short drying time and at the same time not reducing activity of sensible catalyst systems, such as e.g. hybrid catalyst systems comprising late transition metal complexes.
Hybrid catalysts systems comprising at least one late transition metal complex, having Fe, Ni, or Co as central atom, and optionally two or more catalysts selected from metallocene complexes, containing Ti, Zr or Hf as central atom, and/or Ziegler-Natta catalysts, in combination with suitable cocatalysts, are known in the state of the art. Hybrid catalyst systems of this type are useful to generate multimodal molecular weight distributions or reactor blends of polymers having different monomer composition.